In recent years, with a rapid growth of information displays, particularly portable data terminals, a small sized, thin, and low electric power consumption type display having a display capacity and display quality compatible to those of the conventional CRTs (cathode ray tubes) has been increasingly demanded.
The conventional liquid crystal displays have been used as display terminals of up to about a 1/100 duty cycle, such as watches and electric calculators, utilizing multiplexing of a TN (twisted nematic) liquid crystal cell having a twist angle of 90.degree.. In view of the principle, it has been believed that it is difficult to increase the number of duty cycles to more than the above value, because it is accompanied by a reduction of display quality.
Thus a supertwisted birefringence effect mode utilizing the birefringence effect of a liquid crystal cell in which the highest possible twist angle to produce bistability, one of electro-optic characteristics of a chiral nematic liquid crystal, is controlled between 180.degree. and 27.degree. has been developed. This mode is referred to an "SBE mode", including supertwisted nematic modes using an aligning organic film, called an STN mode and an HBE mode.
T.J. Scheffer & J. Nehling disclosed in Appl. Phys. Lett., 45, 1021 (1984) that the above SBE mode can provide a matrix display in which a display device is driven in the same driving manner as that in a matrix display of the conventional 90.degree. twisted TN mode, i.e., the fast scan addressing mode responding to the effective value of applied voltage. It is also reported that the SBE mode display has a much higher contrast and a much wider viewing angle than the 90.degree. twisted TN display.
In connection with a liquid crystal material, to make a voltage-transmittance curve of the conventional 90.degree. twisted nematic mode display steep, it is necessary to minimize an elastic constant ratio K.sub.33 /K.sub.11 of a liquid crystal material to be used (see, for example, G. Bauer, The Physics and Chemistry of Liquid Crystal Devices (edited by G.J. Sprokel), pp. 61-78 (1980)). In the SBE mode, however, to make the voltage transmittance curve steep, it is necessary to make K.sub.33 /K.sub.11 as large as possible unlike in the 90.degree. TN mode, and K.sub.33 /K.sub.22 is desirable to be larger (see, for example, C.M. Waters, Design of Highly Multiplexed Liquid Crystal Dye Displays, Mol. Cryst. Liq. Cryst., 1985, Vol. 123, pp. 303-319).
K.sub.11, K.sub.22 and K.sub.33 as used herein refer to splay, twist, and bend elastic constants, respectively, in the Frank's continuum theory (F. Frank, Disc Faraday Soc., 25, p. 19 (1958)).
As described above, a physical property value required for making a voltage-transmittance curve steep, i.e., elastic constant ratio K.sub.33 /K.sub.11 is in a completely reverse relationship between the conventional 90.degree. TN and SBE modes. For this reason, liquid crystal compositions conventionally used in the 90.degree. TN mode are generally unsuitable for use to make a voltage-transmittance curve steep in the SBE mode and cannot be used in the SBE mode display as they are.
The main characteristics required for liquid crystal compositions to be suitable for use in SBE mode displays are:
(1) a voltage-transmittance curve is steep (steepness of voltage-transmittance curve);
(2) a nematic-isotropic liquid phase transition temperature, i.e., clearing point (NI) is high;
(3) refractive index anisotropy (An) can be suitably controlled depending on a cell thickness (d); and
(4) viscosity (.eta.) is low.
The characteristic (1) above is necessary for increasing display contrast of a liquid crystal display of the SBE mode.
The characteristic (2) is needed to avoid coloring of the display due to temperature dependency of .DELTA.n in the SBE mode; the clearing point is desired to be as high as possible. The reason for this is that the value of .DELTA.n generally decreases gradually as the temperature increases; that is, it is generally a gradually decreasing curve from the lower temperature side to the higher temperature side, and in the vicinity of the clearing point, it begins to decrease markedly, leading to a great change in the optical path (.DELTA.n.multidot.d) and further to a change in color of the display. At the clearing point where .DELTA.n reaches 0, the liquid crystal composition becomes an isotropic liquid, losing its function as a liquid crystal. From a practical point of view, the clearing point is preferably at least 85.degree. C.
The characteristic (3) is necessary for increasing the degree of freedom of d of a liquid crystal display cell. The reason for this is that as the SBE mode employs interference color for display, the color being produced by the birefringence effect due to an optical path (.DELTA.n.multidot.d=constant) of birefringence, unlike the conventional TN mode, it is desirable that the .DELTA.n of a liquid crystal composition can be adjusted to a suitable value depending on d.
The characteristic (4) is particularly effective in shortening the response time in a liquid crystal cell of the SBE mode. With regard to the response time in the TN mode, it is known that both rising and falling times are in proportion to .DELTA..multidot.d.sup.2. This relationship is applicable to the SBE mode as well.
In fact, however, the conventional liquid crystal compositions fail to satisfy the above requirements; a liquid crystal composition satisfying the above requirements and thus suitable for use in displays of the SBE mode has not been found heretofore.